Method of purifying dimethyl sulfoxide

ABSTRACT

A method of purifying dimethyl sulfoxide includes distilling a dimethyl sulfoxide-containing liquid in the presence of sodium carbonate under a reduced pressure in an inert gas atmosphere to distill out dimethyl sulfoxide, an amount of the sodium carbonate with respect to 100 g of pure dimethyl sulfoxide in a residual liquid after the distillation being 6 times or more the amount of the sodium carbonate with respect to 100 g of the pure dimethyl sulfoxide in the dimethyl sulfoxide-containing liquid before the distillation.

TECHNICAL FIELD

This disclosure relates to a method of purifying dimethyl sulfoxide(DMSO).

BACKGROUND

Dimethyl sulfoxide is widely used in industries as a solvent forpolymerization of polymers and spinning polymer fibers. Additionally,recovery and reuse of dimethyl sulfoxide used once are widely performedin industry, and require steps of heating and distilling to purify.

It is, however, known that dimethyl sulfoxide is relatively thermallyunstable and slightly decomposed when distilled under atmosphericpressure. In manufacturing or recovering dimethyl sulfoxide bydistillation, contaminating a partially decomposed decomposition productin dimethyl sulfoxide reduces efficiency of dimethyl sulfoxide as asolvent. Thus, distillation of dimethyl sulfoxide is often performedunder reduced pressure at 100° C. or less.

If dimethyl sulfoxide can be distilled, for example, at a hightemperature of 110° C. or more, it is unnecessary to use high vacuum indistillation and, furthermore, no load is applied to a decompressiondevice. Thus, facility for distillation can be simplified, which isindustrially preferable.

There are known methods of adding a metal hydroxide such as sodiumhydroxide or potassium hydroxide as an inhibitor for dimethyl sulfoxidedecomposition (see JP-B-S43-3765, JP-B-S38-20721 and JP-A-2015-145359).The amount of addition of sodium hydroxide, potassium hydroxide or thelike is limited to 0.003 to 0.5%, and adding in an amount of 1% or morepromotes dimethyl sulfoxide decomposition.

In JP '765, the amount of a decomposition product quantified asformaldehyde after heating at 150° C. for 10 hours was 0.032% when nometal hydroxide was added, 0.054% when 1% of potassium hydroxide wasadded, and 0.052% when 1% of sodium hydroxide was added.

When purifying by distilling dimethyl sulfoxide, the purified dimethylsulfoxide is distilled out of the system by the distillation. Inaddition, water contained in a dimethyl sulfoxide-containing liquid, asolvent having a lower boiling point than dimethyl sulfoxide, unreactedmonomers in polymerization, and impurities such as a decompositionproduct of dimethyl sulfoxide are distilled out of the system bydistillation. As a result, when a metal hydroxide such as sodiumhydroxide or potassium hydroxide as an inhibitor for dimethyl sulfoxidedecomposition is added, the metal hydroxide such as sodium hydroxide orpotassium hydroxide remains at a bottom of a distillation column duringpurification as the distillation and purification of dimethyl sulfoxideproceed. This increases concentration of the metal hydroxide containedin a residual liquid after the distillation. In distilling dimethylsulfoxide, even if a metal hydroxide is added at low concentration,dimethyl sulfoxide decomposition is rather promoted when dimethylsulfoxide is distilled out and the concentration of the metal hydroxidereaches 1% or more. Thus, there has been a problem in that purity isreduced due to contaminating a decomposition product of dimethylsulfoxide in distilled dimethyl sulfoxide.

There has been a desire for a dimethyl sulfoxide-purifying method thatenables high purity dimethyl sulfoxide to be obtained safely, forexample, at a high temperature of 110° C. or more and even when adecomposition inhibitor is highly concentrated in distilling andpurifying dimethyl sulfoxide.

It could therefore be helpful to provide a method of purifying dimethylsulfoxide to obtain high purity dimethyl sulfoxide.

SUMMARY

We provide a method of purifying dimethyl sulfoxide including distillinga dimethyl sulfoxide-containing liquid in the presence of sodiumcarbonate in an inert gas atmosphere to distill out dimethyl sulfoxide,in which an amount of the sodium carbonate with respect to 100 g of puredimethyl sulfoxide in a residual liquid after the distillation is 6times or more the amount of the sodium carbonate with respect to 100 gof pure dimethyl sulfoxide in the dimethyl sulfoxide-containing liquidbefore the distillation.

The method of purifying dimethyl sulfoxide inhibits decomposition ofdimethyl sulfoxide even when the concentration of sodium carbonateincreases from low to high so that high purity dimethyl sulfoxide can beobtained by distillation.

The amount of dimethyl sulfoxide decomposition is small not only in adistillate after the distillation (a main distillate and an earlydistillate) but also in a residual liquid after the distillation.Dimethyl sulfoxide decomposition is small in the residual liquid afterthe distillation or even in a liquid during the distillation so thathigh purity dimethyl sulfoxide can be obtained by the distillation.

The amount of dimethyl sulfoxide decomposition is small, which iscalculated from dimethyl sulfoxide purity in a mixed liquid of thedistillate (the main distillate and the early distillate) afterdistillation and the residual liquid after distillation. Thus, thesodium carbonate used in the dimethyl sulfoxide-purifying methodeffectively inhibits dimethyl sulfoxide decomposition.

The method of purifying dimethyl sulfoxide enables distillation to beperformed at high temperature so that the method does not require anyhigh vacuum facility and enables purification at low cost. Since sodiumcarbonate is safe, operation can be performed safer as compared toconventional purification methods using dangerous substances.

Dimethyl sulfoxide obtained by the method of purifying dimethylsulfoxide can be used as solvents in steps of polymerizing and spinningpolymers such as polyacrylonitrile, cellulose, polyimide, polysulfone,and polyurethane, stripping liquids for photoresists that are electronicmaterials, solvents used in synthesizing pharmaceuticals andagrochemicals, removing and cleaning liquids for lens molds and thelike, or liquids used in paint stripping.

DETAILED DESCRIPTION

We provide a method of purifying dimethyl sulfoxide, includingdistilling a dimethyl sulfoxide-containing liquid in the presence ofsodium carbonate in an inert gas atmosphere to distill out dimethylsulfoxide, in which an amount of the sodium carbonate with respect to100 g of pure dimethyl sulfoxide in a residual liquid after thedistillation is 6 times or more the amount of the sodium carbonate withrespect to 100 g of the pure dimethyl sulfoxide in the dimethylsulfoxide-containing liquid before the distillation.

The sodium carbonate may be either an anhydride or hydrate, and thehydrate is preferably a monohydrate or decahydrate that are easilyavailable.

The sodium carbonate may be added in powder or solid form as it is ormay be added as an aqueous solution. When added as an aqueous solution,the sodium carbonate can be automatically and continuously charged at auniform concentration in a distillation apparatus, which is thereforepreferable in terms of safety. The concentration of the sodium carbonatewhen prepared into an aqueous solution can be increased up to aconcentration saturated at a temperature used. When adding a sodiumcarbonate as an aqueous solution, the amount of the sodium carbonate tobe added is preferably 0.1 to 35 g, more preferably 0.2 to 30 g, stillmore preferably 1 to 30 g, and most preferably 10 to 25 g, with respectto 100 g of water.

Solubility of sodium carbonate into 100 g of water is 22 g at 20° C.,which is high. Accordingly, when sodium carbonate concentration isincreased by distillation and sodium carbonate crystals are deposited,the crystals in the column can be easily removed even in removing bycleaning with water due to its high water solubility.

The dimethyl sulfoxide-containing liquid may be an impurity-free and100% dimethyl sulfoxide liquid. Additionally, the dimethylsulfoxide-containing liquid may be a liquid containing a very small orsmall amount of impurity (impurities). Furthermore, the dimethylsulfoxide-containing liquid may contain a large amount of a liquid otherthan dimethyl sulfoxide.

Since it is costly to distill and remove an impurity (impurities), thedimethyl sulfoxide-containing liquid before distillation preferablycontains pure dimethyl sulfoxide in an amount of 10% by weight or more.The pure dimethyl sulfoxide refers to dimethyl sulfoxide with a purityof 100%. The dimethyl sulfoxide-containing liquid before distillationcontains the pure dimethyl sulfoxide in an amount of more preferably 20%by weight or more, and still more preferably contains the pure dimethylsulfoxide in an amount of from 30 to 100% by weight.

The dimethyl sulfoxide-containing liquid before distillation may containwater other than dimethyl sulfoxide. The amount of the water in thedimethyl sulfoxide-containing liquid before distillation is preferably0.01 to 900 g, more preferably 0.1 to 400 g, still more preferably 1 to250 g, and still further more preferably 5 to 100 g, with respect to 100g of the pure dimethyl sulfoxide in the liquid.

When the dimethyl sulfoxide-containing liquid contains an impurityand/or the like that hinder(s) distillation such as a resin component,an insoluble substance, a component that tends to be gelled whenconcentrated, an acid or a strong alkali, and/or a component that reactswith dimethyl sulfoxide, it is preferable to remove, separate,inactivate, or neutralize the impurity and/or the like by previouslyperforming filtering, adsorption and separation, addition of anactivated carbon, an ion-exchange resin or a base or the like.

When distilling and purifying by adding a decomposition inhibitor in thedimethyl sulfoxide-containing liquid, purified dimethyl sulfoxide isdistilled out of the system by the distillation. Additionally, watercontained in the dimethyl sulfoxide-containing liquid, a solvent havinga lower boiling point than dimethyl sulfoxide, unreacted monomers inpolymerization, and impurities such as a decomposition product ofdimethyl sulfoxide are distilled out of the system by the distillation.As a result, as distillation and purification of dimethyl sulfoxideproceed, the decomposition inhibitor remains at the bottom of thedistillation column during the purification, increasing theconcentration of the decomposition inhibitor in a residual liquid duringthe distillation. When sodium hydroxide or potassium carbonate is usedas the decomposition inhibitor, even if added at low concentration,dimethyl sulfoxide decomposition is promoted when dimethyl sulfoxide isdistilled out and the concentration of the decomposition inhibitorincreases. Then, a decomposition product of dimethyl sulfoxide iscontaminated in distilled dimethyl sulfoxide, thereby reducing dimethylsulfoxide purity.

On the other hand, sodium carbonate inhibits decomposition of dimethylsulfoxide even when highly concentrated by distilling-out of dimethylsulfoxide so that high purity dimethyl sulfoxide can be obtained.

The amount of the sodium carbonate added at a start of distillation ispreferably 0.0005 to 1.0 g, and more preferably 0.001 to 0.5 g withrespect to 100 g of the pure dimethyl sulfoxide in the liquid.

The amount of the sodium carbonate with respect to 100 g of the puredimethyl sulfoxide in the residual liquid after the distillation is 6times or more the amount of the sodium carbonate with respect to 100 gof the pure dimethyl sulfoxide in the dimethyl sulfoxide-containingliquid before the distillation.

The phrase “the residual liquid after the distillation” refers to adimethyl sulfoxide-containing liquid left at the bottom of adistillation column or in a flask at the end of distillation withoutbeing distilled when distilling the dimethyl sulfoxide-containing liquidin a distillation facility.

As long as stirring is possible, the concentration of the sodiumcarbonate in a residual liquid after distillation can be increased.

When the amount of the sodium carbonate with respect to 100 g of thepure dimethyl sulfoxide in the residual liquid after the distillation isbelow 6 times the amount of the sodium carbonate with respect to 100 gof the pure dimethyl sulfoxide in the dimethyl sulfoxide-containingliquid before the distillation, dimethyl sulfoxide recovery rate is low,thus increasing cost for purifying dimethyl sulfoxide.

The amount of the sodium carbonate with respect to 100 g of the puredimethyl sulfoxide in the residual liquid after the distillation ispreferably 6 to 1000 times, more preferably 10 to 500 times, and stillmore preferably 20 to 200 times as much as the amount of the sodiumcarbonate with respect to 100 g of the pure dimethyl sulfoxide in thedimethyl sulfoxide-containing liquid before the distillation.

The amount of the sodium carbonate after the distillation is preferablyconcentrated to 0.01 to 100 g, and more preferably to 0.1 to 85 g, withrespect to 100 g of the pure dimethyl sulfoxide in the residual liquidafter the distillation. When the amount of the sodium carbonate afterthe distillation is 100 g or more, slurry of the residual liquid may behardened at the bottom of the distillation column, which may causedifficulty in stirring.

Regarding timing of sodium carbonate addition in the dimethylsulfoxide-containing liquid, the sodium carbonate may be added beforethe distillation or may be added after distilling away an impurity(impurities) having a lower boiling point than dimethyl sulfoxide toperform the distillation. Additionally, sodium carbonate deposited afterthe distillation may be discarded or recovered and reused.

Dimethyl sulfoxide is distilled in an inert gas atmosphere. The term“inert gas atmosphere” means a nitrogen, carbon dioxide, helium, orargon atmosphere, and may be composed of one kind of gas or a mixed gascomposed of two or more gases. The inert gas atmosphere is preferably anitrogen atmosphere. When distilled in air, dimethyl sulfoxide is easilydecomposed.

Dimethyl sulfoxide is distilled more preferably in an inert gasatmosphere and under atmospheric to reduced pressure. When there is asmall difference in boiling point between an impurity to be desirablyremoved and dimethyl sulfoxide, the degree of pressure reduction is notvery lowered to increase the difference in boiling point between theimpurity and dimethyl sulfoxide, thereby facilitating removal of theimpurity.

When distilling under atmospheric pressure, temperature during thedistillation is preferably 160° C. to 200° C., more preferably 170° C.to 195° C., still more preferably 180° C. to 194° C., and still furthermore preferably 189° C. to 193° C., whereby no load is applied to theapparatus, and also the facility for distillation is simplified, whichis industrially preferable.

When distilling under reduced pressure, distillation is performedpreferably at 10 to 750 Torr, and more preferably 15 to 730 Torr.

When distilling under reduced pressure, temperature during thedistillation is preferably 108° C. to 180° C., more preferably 120° C.to 170° C., and still more preferably 131° C. to 160° C.

The purity of dimethyl sulfoxide is measured by gas chromatography witha capillary column.

The purity of dimethyl sulfoxide purified is analyzed by the gaschromatography using a capillary column and represented by area %. Thepurity thereof is preferably 99.990% or more, more preferably 99.991% ormore, and still more preferably 99.992% or more.

The amount of decomposition (area %) of dimethyl sulfoxide will bedefined as below. The dimethyl sulfoxide-containing liquid beforedistillation is defined as “charged liquid.” A liquid obtained by mixinga distillate after distillation (including a main distillate and, ifany, an early distillate) and a residual liquid after the distillationis defined as “post-distillation mixed liquid.” The purity (area %) ofdimethyl sulfoxide in each of the charged liquid and the“post-distillation mixed liquid” was measured by the gas chromatography,and the amount of decomposition of dimethyl sulfoxide was obtained bythe following calculation equation:

Amount of decomposition of dimethyl sulfoxide (area %)=purity in chargedliquid (area %)−purity in post-distillation mixed liquid (area %).

The amount of decomposition of dimethyl sulfoxide is preferably 0.009area % or less, more preferably 0.008 area % or less, and still morepreferably 0.007 area % or less.

In the method of purifying dimethyl sulfoxide, distillation isapplicable to both of batch distillation and continuous distillation,and the distillation column may be a single column, a composite column,or a combination of two or more distillation columns. When performing acontinuous distillation, a sodium carbonate aqueous solution ispreferably continuously supplied before the distillation column(s).

As for the number of theoretical plates of the distillation column(s),preferred is/are distillation column(s) with 1 to 50 theoretical plates,and suitably, more preferred is/are distillation column(s) with 3 to 40theoretical plates.

The dimethyl sulfoxide-containing liquid may be a dimethylsulfoxide-containing reaction liquid obtained in a step of synthesizingby oxidation or the like of dimethyl sulfide, a dimethylsulfoxide-containing waste liquid used in a step of polymerization orspinning of a polymer such as polyacrylonitrile, cellulose, polyimide,polysulfone, or polyurethane, a dimethyl sulfoxide-containing wasteliquid used as a stripping liquid for a photoresist that is anelectronic material, a dimethyl sulfoxide-containing waste liquid usedas a solvent for synthesizing a pharmaceutical or agrochemical, adimethyl sulfoxide-containing waste liquid used as a removing andcleaning liquid for a lens mold or the like, a dimethylsulfoxide-containing waste liquid used as a paint stripping liquid orthe like.

EXAMPLES

Our methods will be specifically described by Examples hereinafter.Various kinds of measurement values used in the Examples and the likewere measured by the following measurement methods.

(1) Dimethyl Sulfoxide Purity (Area %)

Dimethyl Sulfoxide Purity was measured by gas chromatography under thefollowing conditions:

Apparatus used: GC-2010 (FID) manufactured by Shimadzu Corporation

Column: DB-WAX, 0.25 mm×60 m, film thickness: 0.25 μm

Carrier gas: He: 165.7 kPa

Column temperature-increasing conditions: 35° C.→7° C./min→140° C.×10min→15° C./min→250° C.×10 min

Inlet temperature: 200° C.

Detector temperature: 250° C.

FID: Air: 400 ml/min, H2: 40 ml/min, Makeup: 30 ml/min

Split ratio: 14

Preparation of analysis sample: samples were filtered through a 0.5 μmPTFE syringe filter

Amount of injection: 1.0 μl.

(2) Amount of Decomposition of Dimethyl Sulfoxide (Area %)

Regarding a dimethyl sulfoxide-containing liquid before distillation (acharged liquid) and a mixed liquid prepared by mixing a distillate afterthe distillation (including a main distillate and, if any, an earlydistillate) and a residual liquid after the distillation (apost-distillation mixed liquid), dimethyl sulfoxide purities (area %)were measured in the same manner as in (1), and then, an amount ofdecomposition of dimethyl sulfoxide was obtained by the followingcalculation equation:

Amount of decomposition of dimethyl sulfoxide (area %)=purity in chargedliquid (area %)−purity in post-distillation mixed liquid (area %).

(3) Amount of Additive

The amount of an additive before distillation is an amount of theadditive with respect to 100 g of pure dimethyl sulfoxide in thedimethyl sulfoxide-containing liquid. In the gas chromatographicmeasurement of dimethyl sulfoxide in (1), neither water nor additive isdetected. The amount of the pure dimethyl sulfoxide in the chargedliquid was obtained by multiplying a value obtained by subtractingamounts of water and the additive from a total liquid amount by dimethylsulfoxide purity in the charged liquid.

The amount of the additive after the distillation is an amount of theadditive with respect to 100 g of pure dimethyl sulfoxide in theresidual liquid after the distillation. The amount of the pure dimethylsulfoxide in the residual liquid after the distillation was obtained bymultiplying a value obtained by subtracting amounts of water and theadditive from an amount of the residual liquid in the flask by dimethylsulfoxide purity in the residual liquid.

(4) Additive Concentration Rate Between Before and After Distillation

Additive concentration rate between before and after the distillationwas obtained by the following calculation equation:

Additive concentration rate between before and afterdistillation=(amount (g) of additive with respect to 100 g of puredimethyl sulfoxide in residual liquid after distillation)/(amount (g) ofadditive with respect to 100 g of pure dimethyl sulfoxide in dimethylsulfoxide-containing liquid before distillation).

EXAMPLE 1

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure &methyl sulfoxide: 719.98 g), 80 g of ion-exchanged water,and, as an additive, 0.0072 g of sodium carbonate (0.001 g with respectto 100 g of the pure dimethyl sulfoxide). After substituting the insideof the flask with nitrogen, a rubber balloon filled with nitrogen wasmounted at a top of the Dimroth condenser to seal the flask. The flaskwas heated in an oil bath maintained at 200° C. and a point in time whendistillation started was considered to be a startup time. Aftercollecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 8 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 0.09 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.997 area % asshown in Table 1 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.903 area % asshown in Table 1. Dirnethyl sulfoxide was hardly decomposed even whenheated for three hours at 147 to 170° C. (temperatures in the flask) inthe early distillation operation and at 170 to 191° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 0.09 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 90 times by the dimethyl sulfoxidedistillation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.994 area %, and the amount ofdecomposition was 0.003%, extremely small.

EXAMPLE 2

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchangedwater, and, as an additive, 0.72 g of sodium carbonate (0.1 g withrespect to 100 g of the pure dimethyl sulfoxide). After substituting theinside of the flask with nitrogen, a rubber balloon filled with nitrogenwas mounted at the top of the Dimroth condenser to seal the flask. Theflask was heated in an oil bath maintained at 200° C., and a point intime when distillation started was considered to be a startup time.After collecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 72 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 1 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 2.5 hours.

Dimethyl sulfoxide purity in the main distillate was 99.995 area % asshown in Table 1 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.964 area % asshown in Table 1. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 147 to 170° C. (temperatures in the flask) inthe early distillation operation and at 171 to 191° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 1 g with respect to 100 g ofthe pure dimethyl sulfoxide in the residual liquid, the sodium carbonatewas concentrated to 10 times by the dimethyl sulfoxide distillation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.991 area %, and the amount ofdecomposition was 0.006%, extremely small.

EXAMPLE 3

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchangedwater, and, as an additive, 7.2 g of sodium carbonate (1 g with respectto 100 g of the pure dimethyl sulfoxide). After substituting the insideof the flask with nitrogen, a rubber balloon filled with nitrogen wasmounted at the top of the Dimroth condenser to seal the flask. The flaskwas heated in an oil bath maintained at 200° C., and a point in timewhen distillation started was considered to be a startup time. Aftercollecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 16 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 82 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillation was 99.994 area % asshown in Table 1 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.892 area % asshown in Table 1. Dimethyl sulfoxide was hardly decomposed even whenheated for 3 hours at 147 to 170° C. (temperatures in the flask) in theearly distillation operation and at 170 to 191° C. (temperatures in theflask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 82 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 82 times by the dimethyl sulfoxidedistillation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.992 area %, and the amount ofdecomposition was 0.005%, extremely small.

Comparative Example 1

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g) and 80 g of ion-exchangedwater without adding any additive. After substituting the inside of theflask with nitrogen, a rubber balloon filled with nitrogen was mountedat the top of the Dimroth condenser to seal the flask. The flask washeated in an oil bath maintained at 200° C., and a point in time whendistillation started was considered to be a startup time. Aftercollecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 11 g (the amount ofwater in the residual liquid: 0.0 g). Heating time from the startup was3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.988 area % asshown in Table 1 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.922 area % asshown in Table As shown in Table 1, dimethyl sulfoxide purity in a mixedliquid of the distillate after the distillation (the main distillate andthe early distillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.987 area %, and the amount ofdecomposition was 0.010% that was larger than Examples from 1 to 3.

Comparative Example 2

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchangedwater, and as an additive, 0.144 g of sodium hydroxide (0.02 g withrespect to 100 g of the pure dimethyl sulfoxide). After substituting theinside of the flask with nitrogen, a rubber balloon filled with nitrogenwas mounted at the top of the Dimroth condenser to seal the flask. Theflask was heated in an oil bath maintained at 200° C., and a point intime when distillation started was considered to be a startup time.After collecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 7 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 2 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.960 area % asshown in Table 1 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.568 area % asshown in Table 1. Dimethyl sulfoxide was slightly decomposed when heatedfor 3 hours at 147 to 170° C. (temperatures in the flask) in the earlydistillation operation and at 170 to 191° C. (temperatures in the flask)in the main distillation operation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.956 area %, and the amount ofdecomposition was 0.041% that was larger than Examples from 1 to 3.

Comparative Example 3

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchangedwater, and as an additive, 7.2 g of potassium carbonate (1 g withrespect to 100 g of the pure dimethyl sulfoxide). After substituting theinside of the flask with nitrogen, a rubber balloon filled with nitrogenwas mounted at the top of the Dimroth condenser to seal the flask. Theflask was heated in an oil bath maintained at 200° C., and a point intime when distillation started was considered to be a startup time.After collecting 100 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 18 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 67 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.984 area % asshown in Table 1 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.617 area % asshown in Table 1. Dimethyl sulfoxide was slightly decomposed when heatedfor 3 hours at 147 to 170° C. (temperatures in the flask) in the earlydistillation operation and at 170 to 191° C. (temperatures in the flask)in the main distillation operation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.975 area %, and the amount ofdecomposition was 0.022% that was larger than Examples from 1 to 3.

Comparative Example 4

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.997area %) (pure dimethyl sulfoxide: 719.98 g), 80 g of ion-exchangedwater, and as an additive, 0.216 g of sodium carbonate (0.03 g withrespect to 100 g of the pure dimethyl sulfoxide). Without performingnitrogen substitution, a rubber balloon filled with air was mounted atthe top of the Dimroth condenser to seal the inside of the flask in theair. The flask was heated in an oil bath maintained at 200° C., in whicha point in time when distillation started was considered to be a startuptime. After collecting 100 ml containing water as an early distillate, amain distillation operation was performed in which the temperature ofthe oil bath was increased to 230° C., and distillate was collecteduntil the amount of a residual liquid in the flask reached 22 g (theamount of water in the residual liquid: 0.0 g) and the amount of theadditive after the distillation reached 0.99 g with respect to 100 g ofthe pure dimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.982 area % asshown in Table 1 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.844 area % asshown in Table 1. Dimethyl sulfoxide was slightly decomposed when heatedfor 3 hours at 147 to 170° C. (temperatures in the flask) in the earlydistillation operation and at 171 to 193° C. (temperatures in the flask)in the main distillation operation.

As shown in Table 1, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.978 area %, and the amount ofdecomposition was 0.019% that was larger than Examples from 1 to 3.

TABLE 1 Purity Addi- in post- Charged liquid Main distillate Residualliquid tive distilla- Amount Atmo- Dimethyl Amount Amount Temper-Dimethyl Dimethyl Amount con- tion of sphere sulfoxide of of aturessulfoxide sulfoxide of centra- mixed decompo- Addi- in purity additivewater in flask purity purity additive tion liquid sition tive system(area %) (g) (g) Pressure (° C.) (area %) (area %) (g) rate (area %)(area %) Ex 1 Sodium Nitrogen 99.997 0.001 11 Atmo- 170-191 99.99799.903 0.09 90 99.994 0.003 carbonate spheric Pressure Ex 2 SodiumNitrogen 99.997 0.1 11 Atmo- 171-191 99.995 99.964 1 10 99.991 0.006carbonate spheric Pressure Ex 3 Sodium Nitrogen 99.997 1 11 Atmo-170-191 99.994 99.892 82 82 99.992 0.005 carbonate spheric Pressure ComNone Nitrogen 99.997 — 11 Atmo- 171-192 99.988 99.922 — — 99.987 0.010Ex 1 spheric Pressure Com Sodium Nitrogen 99.997 0.02 11 Atmo- 170-19199.960 99.568 2 100  99.956 0.041 Ex 2 hydroxide spheric Pressure ComPotassium Nitrogen 99.997 1 11 Atmo- 170-191 99.984 99.617 67 67 99.9750.022 Ex 3 carbonate spheric Pressure Com Sodium Air 99.997 0.03 11Atmo- 171-193 99.982 99.844 0.99 33 99.978 0.019 Ex 4 carbonate sphericPressure Amount of additive is the amount of an additive with respect to100 g of the pure dimethyl sulfoxide in the liquid. Amount of water incharged liquid is the amount of water with respect to 100 g of the puredimethyl sulfoxide in the liquid.

In Examples from 1 to 3, high purity dimethyl sulfoxide was obtained asthe main distillates. On the other hand, the dimethyl sulfoxide puritiesin the main distillates obtained in Comparative Examples from 1 to 4were lower than those in Examples from 1 to 3.

EXAMPLE 4

A 10-L four-necked flask equipped with a distillation column filled witha structured packing, a distillate receiver, a stirrer, a thermometer,and a Dimroth condenser provided at the top of the distillation columnwas charged with 4830 g of dimethyl sulfoxide (purity: 99.998 area %)(pure dimethyl sulfoxide: 4829.9 g), 1998 g of ion-exchanged water, and,as an additive, 1.4 g of sodium carbonate (0.03 g with respect to 100 gof the pure dimethyl sulfoxide). After substituting the inside of theflask with nitrogen, a rubber balloon filled with nitrogen was mountedat the top of the Dimroth condenser to seal the flask. The flask washeated in an oil bath maintained at from 145 to 191° C. under a pressureof from 20 to 720 Torr, and a point in time when distillation startedwas considered to be a startup time. After collecting 2289 g containingwater as an early distillate, a main distillation operation wasperformed in which the flask was heated in an oil bath at from 153 to192° C. under a pressure of 20 Torr, and main distillate during a timewhen the temperature in the flask ranged from 110 to 112° C. wascollected until the amount of a residual liquid in the flask reached 688g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 0.2 g with respect to 100 gof the pure dimethyl sulfoxide. Heating time from the startup was 12hours.

Dimethyl sulfoxide purity in the main distillate was 99.999 area % asshown in Table 2 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.975 area % asshown in Table 2. Even when heated, dimethyl sulfoxide was hardlydecomposed. Since the amount of the sodium carbonate in the residualliquid was 0.2 g with respect to 100 g of the pure dimethyl sulfoxide inthe residual liquid, the sodium carbonate was concentrated to 7 times bythe dimethyl sulfoxide distillation.

As shown in Table 2, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate) and the residualliquid after the distillation (a post-distillation mixed liquid) was99.992 area % and the amount of decomposition was 0.006%, extremelysmall. In Example 4, high purity dimethyl sulfoxide was obtained as themain distillate.

TABLE 2 Purity Charged liquid Main distillate Residual liquid in post-Amount Dimethyl Amount Amount Temper- Dimethyl Dimethyl Amount distilla-of sulfoxide of of atures sulfoxide sulfoxide of Additive tion mixeddecompo- purity additive water in flask purity purity additiveconcentra- liquid sition Additive (area %) (g) (g) Pressure (° C.) (area%) (area %) (g) tion rate (area %) (area %) Ex 4 Sodium 99.998 0.03 41Reduced 110-112 99.999 99.975 0.2 7 99.992 0.006 carbonate pressureAmount of additive is the amount of an additive with respect to 100 g ofthe pure dimethyl sulfoxide in the liquid. Amount of water in chargedliquid is the amount of water with respect to 100 g of the pure dimethylsulfoxide in the liquid.

EXAMPLE 5

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 400 g of dimethyl sulfoxide (purity: 99.995area %) (pure dimethyl sulfoxide: 399.98 g), 400 g of ion-exchangedwater, and as an additive, 0.004 g of sodium carbonate (0.001 g withrespect to 100 g of the pure dimethyl sulfoxide). After substituting theinside of the flask with nitrogen, a rubber balloon filled with nitrogenwas mounted at the top of the Dimroth condenser to seal the flask. Theflask was heated in an oil bath maintained at 220° C., and a point intime when distillation started was considered to be a startup time.After collecting 500 ml containing water as an early distillate, a maindistillation operation was performed in which the temperature of the oilbath was increased to 230° C., and distillate was collected until theamount of a residual liquid in the flask reached 9 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 0.044 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.994 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.932 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 3 hours at 108 to 192° C. (temperatures in the flask) in theearly distillation operation and at 192 to 193° C. (temperatures in theflask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 0.044 g with respect to 100g of the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 44 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.992 area %, and the amount ofdecomposition was 0.003%, extremely small.

EXAMPLE 6

Example 6 was performed in the same manner as Example 5 except that theamount of sodium carbonate added was changed to 0.024 g (0.006 g withrespect to 100 g of the pure dimethyl sulfoxide), and distillate wascollected until the amount of a residual liquid in the flask reached 32g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 0.075 g with respect to 100g of the pure dimethyl sulfoxide. Heating time from the startup was 2.5hours.

Dimethyl sulfoxide purity in the main distillate was 99.992 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.964 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 108 to 192° C. (temperatures in the flask) inthe early distillation operation and at 192 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 0.075 g with respect to 100g of the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 13 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.988 area %, and the amount ofdecomposition was 0.007%, extremely small.

EXAMPLE 7

Example 7 was performed in the same manner as Example 5 except that theamount of sodium carbonate added was changed to 0.08 g (0.02 g withrespect to 100 g of pure dimethyl sulfoxide), and distillate wascollected until the amount of a residual liquid in the flask reached 34g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 0.24 g with respect to 100 gof the pure dimethyl sulfoxide. Heating time from the startup was 2.5hours.

Dimethyl sulfoxide purity in the main distillate was 99.991 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.965 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 108 to 192° C. (temperatures in the flask) inthe early distillation operation and at 192 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 0.24 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 12 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.988 area %, and the amount ofdecomposition was 0.007%, extremely small.

EXAMPLE 8

Example 8 was performed in the same manner as Example 5 except that theamount of sodium carbonate added was changed to 0. 4 g (0.1 g withrespect to 100 g of pure dimethyl sulfoxide), and distillate wascollected until the amount of a residual liquid in the flask reached 28g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 1.4 g with respect to 100 gof the pure dimethyl sulfoxide. Heating time from the startup was 2.5hours.

Dimethyl sulfoxide purity in the main distillate was 99.991 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.956 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 108 to 192° C. (temperatures in the flask) inthe early distillation operation and at 192 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 1.4 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 14 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.987 area %, and the amount ofdecomposition was 0.008%, extremely small.

EXAMPLE 9

Example 9 was performed in the same manner as Example 5 except that theamount of sodium carbonate added was changed to 4 g (1 g with respect to100 g of pure dimethyl sulfoxide), and distillate was collected untilthe amount of a residual liquid in the flask reached 42 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 11 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 2.5 hours.

Dimethyl sulfoxide purity in the main distillate was 99.995 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.985 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 108 to 192° C. (temperatures in the flask) inthe early distillation operation and at 192 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 11 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 11 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.992 area %, and the amount ofdecomposition was 0.003%, extremely small.

EXAMPLE 10

Example 10 was performed in the same manner as Example 5 except that theamount of sodium carbonate added was changed to 4 g (1 g with respect to100 g of pure dimethyl sulfoxide), and distillate was collected untilthe amount of a residual liquid in the flask reached 17 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 31 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.994 area % asshown in Table 3 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.968 area % asshown in Table 3. Dimethyl sulfoxide was hardly decomposed even whenheated for 3 hours at 108 to 192° C. (temperatures in the flask) in theearly distillation operation and at 192 to 193° C. (temperatures in theflask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 31 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 31 times by the dimethyl sulfoxidedistillation.

As shown in Table 3, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.992 area %, and the amount ofdecomposition was 0.003%, extremely small.

Comparative Example 5

Comparative Example 5 was performed in the same manner as Example 5except for not adding sodium carbonate, and distillate was collecteduntil the amount of a residual liquid in the flask reached 23 g (theamount of water in the residual liquid: 0.0 g). Heating time from thestartup was 3 hours.

Dimethyl sulfoxide purity in the main distillate was 99.978 area % asshown in Table 3 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.961 area % asshown in Table 3. Dimethyl sulfoxide was slightly decomposed when heatedfor 3 hours at 108 to 192° C. (temperatures in the flask) in the earlydistillation operation and at 192 to 193° C. (temperatures in the flask)in the main distillation operation. As shown in Table 3, dimethylsulfoxide purity in a mixed liquid of the distillate after thedistillation (the main distillate and the early distillate) and theresidual liquid after the distillation (a post-distillation mixedliquid) was 99.979 area %, and the amount of decomposition was 0.016%that was larger than Examples 5 to 10. Additionally, as a decompositionproduct of the dimethyl sulfoxide, a small amount of polymer, whichseemed to be polyacetal, was deposited on the tool after thedistillation.

Comparative Example 6

Comparative Example 6 was performed in the same manner as Example 5except that the kind of the additive was changed to sodium hydroxide andthe amount of the additive added was changed to 0.08 g (0.02 g withrespect to 100 g of pure dimethyl sulfoxide), and distillate wascollected until the amount of a residual liquid in the flask reached 35g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 0.23 g with respect to 100 gof the pure dimethyl sulfoxide. Heating time from the startup was 2.5hours.

Dimethyl sulfoxide purity in the main distillate was 99.980 area % asshown in Table 3 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.926 area % asshown in Table 3. Dimethyl sulfoxide was slightly decomposed when heatedfor 2.5 hours at 108 to 192° C. (temperatures in the flask) in the earlydistillation operation and at 192 to 193° C. (temperatures in the flask)in the main distillation operation. As shown in Table 3, dimethylsulfoxide purity in a mixed liquid of the distillate after thedistillation (the main distillate and the early distillate) and theresidual liquid after the distillation (a post-distillation mixedliquid) was 99.971 area %, and the amount of decomposition was 0.024%that was larger than Examples from 5 to 10.

Comparative Example 7

Comparative Example 7 was performed in the same manner as Example 5except that the kind of the additive was changed to sodium hydroxide andthe amount of the additive added was changed to 0.4 g (0.1 g withrespect to 100 g of pure dimethyl sulfoxide), and distillate wascollected until the amount of a residual liquid in the flask reached 32g (the amount of water in the residual liquid: 0.0 g) and the amount ofthe additive after the distillation reached 1.3 g with respect to 100 gof the pure dimethyl sulfoxide. Heating time from the startup was 2.5hours.

Dimethyl sulfoxide purity in the main distillate was 99.972 area % asshown in Table 3 which indicated reduced dimethyl sulfoxide purity.

Dimethyl sulfoxide purity in the residual liquid was 99.910 area % asshown in Table 3. Dimethyl sulfoxide was slightly decomposed when heatedfor 2.5 hours at 108 to 192° C. (temperatures in the flask) in the earlydistillation operation and at 192 to 193° C. (temperatures in the flask)in the main distillation operation. As shown in Table 3, dimethylsulfoxide purity in a mixed liquid of the distillate after thedistillation (the main distillate and the early distillate) and theresidual liquid after the distillation (a post-distillation mixedliquid) was 99.963 area %, and the amount of decomposition was 0.032%that was larger than Examples from 5 to 10.

TABLE 3 Purity Charged liquid Main distillate Residual liquid in post-Amount Dimethyl Amount Amount Temper- Dimethyl Dimethyl Amount distilla-of sulfoxide of of atures sulfoxide sulfoxide of Additive tion mixeddecompo- purity additive water in flask purity purity additiveconcentra- liquid sition Additive (area %) (g) (g) Pressure (° C.) (area%) (area %) (g) tion rate (area %) (area %) Ex 5 Sodium 99.995 0.001 100Atmospheric 192-193 99.994 99.932 0.044 44 99.992 0.003 carbonatepressure Ex 6 Sodium 99.995 0.006 100 Atmospheric 192-193 99.992 99.9640.075 13 99.988 0.007 carbonate pressure Ex 7 Sodium 99.995 0.02 100Atmospheric 192-193 99.991 99.965 0.24 12 99.988 0.007 carbonatepressure Ex 8 Sodium 99.995 0.1 100 Atmospheric 192-193 99.991 99.9561.4 14 99.987 0.008 carbonate pressure Ex 9 Sodium 99.995 1 100Atmospheric 192-193 99.995 99.985 11 11 99.992 0.003 carbonate pressureEx 10 Sodium 99.995 1 100 Atmospheric 192-193 99.994 99.968 31 31 99.9920.003 carbonate pressure Com None 99.995 — 100 Atmospheric 192-19399.978 99.961 — — 99.979 0.016 Ex 5 pressure Com Sodium 99.995 0.02 100Atmospheric 192-193 99.980 99.926 0.23 12 99.971 0.024 Ex 6 hydroxidepressure Com Sodium 99.995 0.1 100 Atmospheric 192-193 99.972 99.910 1.313 99.963 0.032 Ex 7 hydroxide pressure Amount of additive is the amountof an additive with respect to 100 g of the pure dimethyl sulfoxide inthe liquid. Amount of water in charged liquid is the amount of waterwith respect to 100 g of the pure dimethyl sulfoxide in the liquid.

In Examples 5 to 10, high purity dimethyl sulfoxide was obtained as themain distillates. On the other hand, the dimethyl sulfoxide purities inthe main distillates obtained in Comparative Examples 5 to 7 were lowerthan those in Examples 5 to 10.

EXAMPLE 11

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.995area %) (pure dimethyl sulfoxide: 719.96 g) and, as an additive, 7.2 gof sodium carbonate (1 g with respect to 100 g of the pure dimethylsulfoxide). After substituting the inside of the flask with nitrogen, arubber balloon filled with nitrogen was mounted at the top of theDimroth condenser to seal the flask. The flask was heated in an oil bathmaintained at 220° C., and a point in time when distillation started wasconsidered to be a startup time. Distillate was collected until theamount of a residual liquid in the flask reached 56 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the additiveafter the distillation reached 15 g with respect to 100 g of the puredimethyl sulfoxide. Heating time from the startup was 2.5 hours.

Dimethyl sulfoxide purity in the main distillate was 99.990 area % asshown in Table 4 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.930 area % asshown in Table 4. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 192 to 193° C. (temperatures in the flask).Since the amount of the sodium carbonate in the residual liquid was 15 gwith respect to 100 g of the pure dimethyl sulfoxide in the residualliquid, the sodium carbonate was concentrated to 15 times by thedimethyl sulfoxide distillation.

As shown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate) and the residualliquid after the distillation (a post-distillation mixed liquid) was99.986 area %, and the amount of decomposition was 0.009%, extremelysmall.

EXAMPLE 12

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver. a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.995area %) (pure dimethyl sulfoxide: 719.96 g). Next, as an additive, 7.2 gof sodium carbonate monohydrate (0.86 g of the sodium carbonate withrespect to 100 g of the pure dimethyl sulfoxide) was charged therein.After substituting the inside of the flask with nitrogen, a rubberballoon filled with nitrogen was mounted at the top of the Dimrothcondenser to seal the flask. The flask was heated in an oil bathmaintained at 220° C., and a point in time when distillation started wasconsidered to be a startup time. After collecting 12 ml containing wateras an early distillate, a main distillation operation was performed inwhich the temperature of the oil bath was increased to 230° C., anddistillate was collected until the amount of a residual liquid in theflask reached 61 g (the amount of water in the residual liquid: 0.0 g)and the amount of the sodium carbonate after the distillation reached11.4 g with respect to 100 g of the pure dimethyl sulfoxide. Heatingtime from the startup was 2.5 hours.

Dimethyl sulfoxide purity in the main distillate was 99.990 area % asshown in Table 4 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.936 area % asshown in Table 4. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.5 hours at 191 to 192° C. (temperatures in the flask) inthe early distillation operation and at 192 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 11.4 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 13 times by the dimethyl sulfoxidedistillation.

As shown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.986 area %, and the amount ofdecomposition was 0.009%, extremely small.

EXAMPLE 13

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.995area %) (pure dimethyl sulfoxide: 719.96 g). Next, as an additive, 7.2 gof sodium carbonate decahydrate (0.37 g of the sodium carbonate withrespect to 100 g of the pure dimethyl sulfoxide) was charged therein.After substituting the inside of the flask with nitrogen, a rubberballoon filled with nitrogen was mounted at the top of the Dimrothcondenser to seal the flask. The flask was heated in an oil bathmaintained at 220° C., and a point in time when distillation started wasconsidered to be a startup time. After collecting 16 ml containing wateras an early distillate, a main distillation operation was performed inwhich the temperature of the oil bath was increased to 230° C., anddistillate was collected until the amount of a residual liquid in theflask reached 82 g (the amount of water in the residual liquid: 0.0 g)and the amount of the sodium carbonate after the distillation reached3.4 g with respect to 100 g of the pure dimethyl sulfoxide. Heating timefrom the startup was 2.8 hours.

Dimethyl sulfoxide purity in the main distillate was 99.990 area % asshown in Table 4 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.951 area % asshown in Table 4. Dimethyl sulfoxide was hardly decomposed even whenheated for 2.8 hours at 187 to 190° C. (temperatures in the flask) inthe early distillation operation and at 191 to 193° C. (temperatures inthe flask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 3.4 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 9 times by the dimethyl sulfoxidedistillation.

As shown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.986 area %, and the amount ofdecomposition was 0.009%, extremely small.

EXAMPLE 14

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 720 g of dimethyl sulfoxide (purity: 99.996area %) (pure dimethyl sulfoxide: 759.97 g). Next, as an additive, therewas charged a sodium carbonate aqueous solution prepared by dissolving7.2 g of sodium carbonate (1 g with respect to 100 g of the puredimethyl sulfoxide) in 40 g of ion-exchanged water. After substitutingthe inside of the flask with nitrogen, a rubber balloon filled withnitrogen was mounted at the top of the Dimroth condenser to seal theflask. The flask was heated in an oil bath maintained at 220° C., and apoint in time when distillation started was considered to be a startuptime. After collecting 140 ml containing water as an early distillate, amain distillation operation was performed in which the temperature ofthe oil bath was increased to 230° C., and distillate was collecteduntil the amount of a residual liquid in the flask reached 56 g (theamount of water in the residual liquid: 0.0 g) and the amount of thesodium carbonate after the distillation reached 15 g with respect to 100g of the pure dimethyl sulfoxide. Heating time from the startup was 2hours.

Dimethyl sulfoxide purity in the main distillate was 99.995 area % asshown in Table 4 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.956 area % asshown in Table 4. Dimethyl sulfoxide was hardly decomposed even whenheated for 2 hours at 162 to 191° C. (temperatures in the flask) in theearly distillation operation and at 192 to 193° C. (temperatures in theflask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 15 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 15 times by the dimethyl sulfoxidedistillation.

As shown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.992 area %, and the amount ofdecomposition was 0.004%, extremely small.

EXAMPLE 15

A 1-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 400 g of dimethyl sulfoxide (purity: 99.996area %) (pure dimethyl sulfoxide: 399.98 g). Next, as an additive, therewas charged a sodium carbonate aqueous solution prepared by dissolving0.4 g of sodium carbonate (0.1 g with respect to 100 g of the puredimethyl sulfoxide) in 400 g of ion-exchanged water. After substitutingthe inside of the flask with nitrogen, a rubber balloon filled withnitrogen was mounted at the top of the Dimroth condenser to seal theflask. Under atmospheric pressure, the flask was heated in an oil bathmaintained at 220° C. and a point in time when distillation started wasconsidered to be a startup time. An amount of 500 ml containing water asan early distillate was collected. Next, as a main distillationoperation, the temperature of the oil bath was decreased to 170° C., andunder a pressure of from 150 to 170 Torr, distillate was collected untilthe amount of a residual liquid in the flask reached 22 g (the amount ofwater in the residual liquid: 0.0 g) and the amount of the sodiumcarbonate after the distillation reached 1.9 g with respect to 100 g ofthe pure dimethyl sulfoxide. Heating time from the startup was 3 hours,

Dimethyl sulfoxide purity in the main distillate was 99.997 area % asshown in Table 4 so that high purity dimethyl sulfoxide was obtained.

Dimethyl sulfoxide purity in the residual liquid was 99.938 area % asshown in Table 4. Dimethyl sulfoxide was hardly decomposed even whenheated for 3 hours at 108 to 193° C. (temperatures in the flask) in theearly distillation operation and at 138 to 140° C. (temperatures in theflask) in the main distillation operation. Since the amount of thesodium carbonate in the residual liquid was 1.9 g with respect to 100 gof the pure dimethyl sulfoxide in the residual liquid, the sodiumcarbonate was concentrated to 19 times by distillation of dimethylsulfoxide.

As shown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate and the earlydistillate) and the residual liquid after the distillation (apost-distillation mixed liquid) was 99.989 area %, and the amount ofdecomposition was 0.007%, extremely small.

Comparative Example 8

Al-L four-necked flask equipped with a Dimroth condenser, a distillatereceiver, a stirrer, and a thermometer necessary for simple distillationoperation was charged with 201 g of dimethyl sulfoxide (purity: 99.996area %) (pure dimethyl sulfoxide: 200.99 g), without charging anyadditive. After substituting the inside of the flask with nitrogen, arubber balloon filled with nitrogen was mounted at the top of theDimroth condenser to seal the flask. The flask was heated in an oil bathmaintained at 220° C., and a point in time when distillation started wasconsidered to be a startup time. When distillate was collected until theamount of a residual liquid in the flask reached 50 g (the amount ofwater in the residual liquid: 0.0 g), dimethyl sulfoxide purity in themain distillate was 99.972 area % as shown in Table 4. Due to thereduction of the dimethyl sulfoxide purity, the distillation wasstopped. Heating time from the startup was 2 hours. Dimethyl sulfoxidepurity in the residual liquid was 99.986 area % as shown in Table 4. Asshown in Table 4, dimethyl sulfoxide purity in a mixed liquid of thedistillate after the distillation (the main distillate) and the residualliquid after the distillation (a post-distillation mixed liquid) was99.975 area %, and the amount of decomposition was 0.021% that waslarger than Example 11.

TABLE 4 Purity Charged liquid Main distillate Residual liquid in post-Amount Dimethyl Amount Amount Temper- Dimethyl Dimethyl Amount distilla-of sulfoxide of of atures sulfoxide sulfoxide of Additive tion mixeddecompo- purity additive water in flask purity purity additiveconcentra- liquid sition Additive (area %) (g) (g) Pressure (° C.) (area%) (area %) (g) tion rate (area %) (area %) Ex 11 Sodium 99.995 1 0Atmospheric 192-193 99.990 99.930 15 15 99.986 0.009 carbonate pressureEx 12 Sodium 99.995 0.86 0.14 Atmospheric 192-193 99.990 99.936 11.4 1399.986 0.009 carbonate pressure Ex 13 Sodium 99.995 0.37 0.63Atmospheric 191-193 99.990 99.951 3.4 9 99.986 0.009 carbonate pressureEx 14 Sodium 99.996 1 5.3 Atmospheric 192-193 99.995 99.956 15 15 99.9920.004 carbonate pressure Ex 15 Sodium 99.996 0.1 100 Reduced 138-14099.997 99.938 1.9 19 99.989 0.007 carbonate pressure Com None 99.996 — 0Atmospheric 192-193 99.972 99.986 — — 99.975 0.021 Ex 8 pressure Amountof additive is the amount of an additive with respect to 100 g of thepure dimethyl sulfoxide in the liquid. Amount of water in charged liquidis the amount of water with respect to 100 g of the pure dimethylsulfoxide in the liquid.

In Examples 11 to 15, high purity dimethyl sulfoxide was obtained as themain distillates. The dimethyl sulfoxide purity obtained in ComparativeExample 8 was lower than Example 11.

The above results showed that our method of purifying dimethyl sulfoxideprovided the high purity dimethyl sulfoxide as the main distillates.

Additionally, as purification proceeded by distilling the dimethylsulfoxide-containing liquid, the decomposition inhibitor remained at abottom of the distillation column during the purification, therebyincreasing concentration of the decomposition inhibitor included in theresidual liquid during distillation. In the use of sodium carbonate asthe decomposition inhibitor, even when dimethyl sulfoxide was distilledout and the sodium carbonate concentration increased, decomposition ofdimethyl sulfoxide was not promoted, and dimethyl sulfoxide purity washigh. On the other hand, in the use of sodium hydroxide or potassiumcarbonate as the decomposition inhibitor, even when added at lowconcentration, decomposition of dimethyl sulfoxide was promoted whendimethyl sulfoxide was distilled out and the decomposition inhibitorconcentration increased. Then, a decomposition product of the dimethylsulfoxide was contaminated into the distilled dimethyl sulfoxide,thereby reducing dimethyl sulfoxide purity.

INDUSTRIAL APPLICABILITY

Dimethyl sulfoxide obtained by our method of purifying dimethylsulfoxide has high purity and can be used as solvents in steps ofpolymerizing and spinning polymers such as polyacrylonitrile, cellulose,polyimide, polysulfone, and polyurethane, stripping liquids forphotoresists that are electronic materials, solvents for synthesizingpharmaceuticals and agrochemicals, removing and cleaning liquids forlens molds and the like, or paint stripping liquids.

1. A method of purifying dimethyl sulfoxide comprising distilling adimethyl sulfoxide-containing liquid in the presence of sodium carbonateunder reduced pressure in an inert gas atmosphere to distill outdimethyl sulfoxide, an amount of the sodium carbonate with respect to100 g of pure dimethyl sulfoxide in a residual liquid after thedistillation being 6 times or more the amount of the sodium carbonatewith respect to 100 g of the pure dimethyl sulfoxide in the dimethylsulfoxide-containing liquid before the distillation.
 2. The methodaccording to claim 1, wherein the distillation is performed by adding,at a start of the distillation, the sodium carbonate in an amount of0.0005 to 1.0 g with respect to 100 g of the pure dimethyl sulfoxide inthe dimethyl sulfoxide-containing liquid.
 3. The method according toclaim 1, wherein the amount of the sodium carbonate with respect to 100g of the pure dimethyl sulfoxide in the residual liquid after thedistillation is 6 to 1000 times the amount of the sodium carbonate withrespect to 100 g of the pure dimethyl sulfoxide in the dimethylsulfoxide-containing liquid before the distillation.
 4. The methodaccording to claim 1, wherein the amount of the sodium carbonate in theresidual liquid after the distillation is 0.01 to 100 g with respect to100 g of the pure dimethyl sulfoxide in the residual liquid after thedistillation.
 5. The method according to claim 1, wherein, after waterin the dimethyl sulfoxide-containing liquid is distilled out as an earlydistillate, the dimethyl sulfoxide is distilled out as a maindistillate.
 6. The method according to claim 1, wherein the purifieddimethyl sulfoxide has a purity of 99.990 area % or more.
 7. The methodaccording to claim 1, wherein the sodium carbonate is used as aninhibitor for dimethyl sulfoxide decomposition.
 8. The method accordingto claim 2, wherein, after water in the dimethyl sulfoxide-containingliquid is distilled out as an early distillate, the dimethyl sulfoxideis distilled out as a main distillate.
 9. The method according to claim3, wherein, after water in the dimethyl sulfoxide-containing liquid isdistilled out as an early distillate, the dimethyl sulfoxide isdistilled out as a main distillate.
 10. The method according to claim 4,wherein, after water in the dimethyl sulfoxide-containing liquid isdistilled out as an early distillate, the dimethyl sulfoxide isdistilled out as a main distillate.
 11. The method according to claim 2,wherein the purified dimethyl sulfoxide has a purity of 99.990 area % ormore.
 12. The method according to claim 3, wherein the purified dimethylsulfoxide has a purity of 99.990 area % or more.
 13. The methodaccording to claim 4, wherein the purified dimethyl sulfoxide has apurity of 99.990 area % or more.
 14. The method according to claim 5,wherein the purified dimethyl sulfoxide has a purity of 99.990 area % ormore.
 15. The method according to claim 2, wherein the sodium carbonateis used as an inhibitor for dimethyl sulfoxide decomposition.
 16. Themethod according to claim 1, wherein the distillation is performed at areduced pressure of 10 to 750 Torr.
 17. The method according to claim 1,wherein the distillation is performed at a reduced pressure of 15 to 730Torr.
 18. The method according to claim 1, wherein the distillation isperformed at a temperature of 108° C. to 180° C.
 19. The methodaccording to claim 1, wherein the distillation is performed at atemperature of 120° C. to 170° C.
 20. The method according to claim 1,wherein the distillation is performed at a. temperature of 131° C. to160° C.